Optical modulation device and display apparatus with three birefringent films each acting as a half waveplate

ABSTRACT

An optical modulation device is constituted by a polarizer; a first film forming a first state and a second state depending on an electric field applied thereto, the first state causing birefringence and the second state not causing birefringence respectively of polarized light from the polarizer, the first film having a thickness set for functioning as a halfwave plate in its first state; and a second film not causing birefringence of light having passed through the second state of the first film but causing birefringence of light having passed through the first state of the first film, the second film having a thickness set for functioning as quarter wave plate or a halfwave plate when the first film is set in its first state. The light from the second film is caused to enter the second film again through a reflection means or a third film selectively forming a first state causing birefringence of light which has caused birefringence and passed through the second film or a second state not causing birefringence of light which has passed through the second film without causing birefringence. The light thus modulated is then caused to enter an analyzer. As a result, an optical modulation giving a large contrast is provided by using a material having a small birefringence effect for the first film or third film.

This application is a division of application Ser. No. 08/266,320, Jun.27, 1994, now U.S. Pat. No. 5,392,142 which is a continuation ofapplication Ser. No. 07/673,070, filed Mar. 21, 1991, now abandoned.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an optical modulation device using amaterial having a refractive index anisotropy, particularly an opticalmodulation device using a ferroelectric chiral smectic liquid crystal,and a display apparatus using the same.

Among optical modulation devices using a ferroelectric chiral smecticliquid crystal, a type of device wherein a thin layer of the liquidcrystal is disposed between two parallel substrates with a very thin gap(e.g., 1-2 microns) therebetween to form a bistable state according tothe surface action of the two substrates (SSFLC: surface-stabilizedferroelectric liquid crystal; Appl. Phys. Lett. 36 (1980), 899), is usedfor various applications because of its high-speed responsiveness,memory characteristic, etc.

The above-mentioned bistability-type ferroelectric liquid crystal deviceprovides two stable states of liquid crystal molecules forming a certainangle from an axial direction (e.g., rubbing direction) on alignmentsurfaces formed by the liquid crystal sides of the two substratessandwiching the thin layer of the liquid crystal. A half of the anglebetween the two stable states is referred to as a tilt angle(hereinafter denoted by θ_(c)). When a voltage is applied in a directionperpendicular to the liquid crystal layer of the liquid crystal device,the ferroelectric liquid crystal is switched from one stable state tothe other stable state. This change corresponds to rotation of anoptical axis of a material having a refractive index anisotropy by anangle 2θ_(c). Accordingly, if polarized light is incident to aferroelectric liquid crystal device of the above-mentioned type having athickness corresponding to the function of a halfwave plate, thebistable two states show polarized light-rotating functions whichmutually differ from each other by 4θ_(c) with respect to the incidentpolarized light. If the above-mentioned ferroelectric liquid crystaldevice is sandwiched between a pair of polarizers (polarizing plates,etc.) disposed in cross nicols or parallel nicols, the ON/OFF ratio ofthe transmitted light quantities (transmittance ratio, contrast) betweenthe two stable states becomes the highest under the condition of:

    4θ.sub.c =90 degrees (i.e., θ.sub.c =22.5 degrees).

However, the above-mentioned tilt angle θ_(c) strongly depends on theliquid crystal material and the property of the alignment surface, sothat any ferroelectric liquid crystal devices obtained heretofore havefailed to provide a sufficient tilt angle θ_(c), thus failing to providea sufficient degree of optical modulation when used as an opticalmodulation device.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical modulationdevice realizing a high-contrast display and a display apparatus usingthe same.

Another object of the present invention is to provide an opticalmodulation device accomplishing a high-transmittance characteristic anda display apparatus using the same.

According to an aspect of the present invention, there is provided anoptical modulation device, comprising:

a polarizer;

a first film forming a first state and a second state depending on anelectric field applied thereto, the first state causing birefringenceand the second state not causing birefringence respectively of polarizedlight from the polarizer, the first film having a thickness set forfunctioning as a halfwave plate in its first state; and

a second film not causing birefringence of light having passed throughthe second state of the first film but causing birefringence of lighthaving passed through the first state of the first film, the second filmhaving a thickness set for functioning as a halfwave plate when thefirst film is set in its first state, the second film being so arrangedthat light having passed through the second film without causingbirefringence is caused to pass through a second state not causingbirefringence and light having passed through the second film whilecausing birefringence is caused to pass through a first state causingbirefringence, followed by passing of the light through an analyzer.

According to another aspect of the present invention, there is providedan optical modulation device, comprising:

a polarizer;

a first film forming a first state and a second state depending on anelectric field applied thereto, the first state causing birefringenceand the second state not causing birefringence respectively of polarizedlight from the polarizer, the first film having a thickness set forfunctioning as a halfwave plate in its first state;

a second film not causing birefringence of light having passed throughthe second state of the first film but causing birefringence of lighthaving passed through the first state of the first film, the second filmhaving a thickness set for functioning as a quarter wave plate when thefirst film is set in its first state;

a reflecting means for reflecting light having passed through the secondfilm again to the second film; and

an analyzer.

According to still another aspect of the present invention, there isprovided an optical modulation device, comprising:

a polarizer;

a first film forming a first state and a second state depending on anelectric field applied thereto, the first state causing birefringenceand the second state not causing birefringence respectively of polarizedlight from the polarizer, the first film having a thickness set forfunctioning as a halfwave plate in its first state;

a second film not causing birefringence of light having passed throughthe second state of the first film but causing birefringence of lighthaving passed through the first state of the first film, the second filmhaving a thickness set for functioning as a quarter wave plate when thefirst film is set in its first state;

a third film selectively forming a first state causing birefringence oflight which has caused birefringence and passed through the second filmor a second state not causing birefringence of light which has passedthrough the second film without causing birefringence, and

an analyzer.

According to a further aspect of the present invention, there isprovided an optical modulation device, comprising:

a polarizer;

a first film forming a first state and a second state different from thefirst state depending on an electric field applied thereto, the firststate causing birefringence and the second state causing or not causingbirefringence respectively of polarized light from the polarizer;

a second film having a thickness set for functioning as a halfwave platefor light having passed through the first film;

a third film having a first state and a second state different from thefirst state depending on an electric field applied thereto depending onan electric field applied thereto, the first state causing birefringenceand the second state causing or not causing birefringence respectivelyof light having passed through the second film; and

an analyzer.

According to a still further aspect of the present invention, there isprovided a display apparatus, comprising:

a light source for emitting indefinitely polarized light;

a polarization beam splitter;

an optical modulation device including (a) a first film forming a firststate and a second state depending on an electric field applied thereto,the first state causing birefringence and the second state not causingbirefringence respectively of polarized light from the polarization beamsplitter, the first film having a thickness for functioning as ahalfwave plate in its first state, (b) a second film not causingbirefringence of light having passed through the second state of thefirst film but causing birefringence of light having passed through thefirst state of the first film, the second film having a thickness setfor functioning as a quarter wave plate when the first film is set inits first state, and (c) a reflection means for reflecting light havingpassed through the second film again to the second film and thepolarization beam splitter; and

a voltage application means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional illustration of an embodiment of thedisplay apparatus according to the invention.

FIGS. 2A and 2B are schematic perspective views for illustrating afunction or action of the invention.

FIG. 3 is a partial sectional view of an embodiment of the opticalmodulation device according to the invention.

FIGS. 4A and 4B are schematic perspective views for illustrating anotherfunction or action of the invention.

FIGS. 5 and 6 are respectively partial sectional views of anotherembodiment of the optical modulation device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 4A and 4B are schematic perspective views for illustrating afunction or action of the present invention.

A liquid crystal device illustrated in FIG. 4A comprises films 11 and 13of an identical bistable ferroelectric liquid crystal having anidentical alignment axis direction and a halfwave plate 12 disposedbetween the films 11 and 13. The long axis directions n₁ and n₃ ofliquid crystal molecules (more strictly, principal axis directions ofindex ellipsoids of liquid crystal molecules) at one stable state (firststable state) of the above-mentioned two ferroelectric liquid crystalfilms 11 and 13 and a principal axis direction n₂ of the index ellipsoidof the halfwave plate 12 are all directed in the same direction. Thethree films 11, 12 and 13 are parallel to each other and each of themhas a function corresponding to that of a halfwave plate with respect toa dominant wavelength. When an electromagnetic wave having a vibratingelectric field in parallel to n₁, n₂ and n₃ is incident to the liquidcrystal device disposed in the above-described manner, the resultantmagnetic waves E₁, E₂ and E₃ (=Eout, outgoing wave) having passed thefilms 11, 12 and 13, respectively, do not change their vibratingelectric fields (Ein/E₁ /E₂ /Eout (=E₃)).

On the other hand, FIG. 4B shows an arrangement of the same device whenthe bistable ferroelectric liquid crystal films 11 and 13 are orientedto the other stable state (second stable state), the liquid crystal longaxes in the ferroelectric liquid crystal films 11 and 13 are rotated by2θ_(c) in the same direction from the vibrating electric field Ein ofthe incident light. As a result, the electric field E₁ of the lighthaving passed through the first liquid crystal film 11 is rotated by4θ_(c) from Ein of the incident light. Then, the electric field E₂ ofthe light having passed through the halfwave plate 12 is caused toassume a direction rotated by -4θ_(c) from the principal axis n₂ of theplate. Finally, the electric field E₃ (=Eout) of the light having passedthrough the second liquid crystal film 13 which has a liquid crystalmolecular long axis n₃ rotated by 6θ_(c) (=4θ_(c) +2θ_(c)) from E₂, iscaused to assume a direction rotated by 6θ_(c) from the liquid crystalmolecular long axis n₃. Consequently, the electric field Eout of theoutgoing light is caused to have a direction which is rotated by 8θ_(c)(=2θ_(c) +6θ_(c)) with respect to the electric field Ein of the incidentlight. This means that it is possible to obtain a rotation angle ofpolarized light which is twice that obtained by a single liquid crystalfilm. Accordingly, only a tilt angle θ_(c) =11.25 degrees (giving 8θ_(c)=90 degrees) is required to provide a maximum on/off ratio(transmittance ratio, contrast).

In this instance, in the present invention, the liquid crystal films 11and 13 may preferably have a thickness d providing an optical phasedifference Δnd (Δn(=n//-n⊥): refractive index anisotropy) ofapproximately 1/2λ(λ:wavelength of light), e.g., 0.9 to 1.1×λ/2, morespecifically e.g., a thickness of 1.2-1.6 microns. Particularly, whenthe liquid crystal films 11 and 13 comprise a chiral smectic liquidcrystal and the thickness is in the range of 1.2-1.6 microns, thehelical structure of the chiral smectic liquid crystal is suppressed toform an alignment state developing bistability.

FIG. 1 schematically illustrates an embodiment of the display apparatuswhich is constituted as a projection-type one including anotherembodiment of the optical modulation device according to the presentinvention. The function of the optical modulation device is illustratedin FIGS. 2A and 2B, and the detailed structure thereof is shown in FIG.3 as a partial sectional view.

Referring to FIG. 1, indefinitely polarized light emitted from a lightsource 16 is reflected by a reflecting shade 17, collimated by acondenser lens 18 and incident to a polarization beam splitter 19, wherea P polarization component is allowed to pass and an S polarizationcomponent is reflected in the perpendicular direction.

The S component is caused to pass through a bistable ferroelectricliquid crystal film 11 functioning as a halfwave plate and a quarterwave plate 14, reflected at a reflection plate 15, and again caused topass through the quarter wave 14 and the bistable ferroelectric liquidcrystal film 11.

As a result, the above-mentioned S polarization component yields a Ppolarization component depending on the state of the bistableferroelectric liquid crystal film 11 and, when it again enters thepolarization beam splitter 19, the S polarization component is reflectedand the P polarization component is allowed to pass to be projected by aprojection lens 10 to form an image on an image projection screen (notshown). In this embodiment, the polarization beam splitter 19 functionsas a polarizer and analyzer.

FIGS. 2A and 2B are enlarged illustrations of the optical modulationdevice including the ferroelectric liquid crystal film 11, the quarterwave plate 14 and the reflection plate 15. Referring to these figures,the incident light is linearly polarized light having a vibratingelectric field Ein. When the long axis n₂ of the ferroelectric liquidcrystal 11 is in one stable state (first stable state) as represented bya dot-and-dash line which is parallel to the refractive principal axisn₄ of the halfwave plate and Ein, no rotation of the polarized lightoccurs. However, when the long axis n₁ of the liquid crystal 11 assumesa direction which is rotated by an angle 2θ_(c) from Ein as shown inFIGS. 2A and 2B, the light having passed through the liquid crystal film11 is caused to have an electric field E₁ which is rotated by 4θ_(c)from Ein as shown in FIG. 2A. Then, the light having passed through thequarter wave plate 14 is converted into circularly polarized lightrepresented by E₂, which is reflected at the reflection plate 15 andagain enters the quarter wave plate 14. Here, the two-times of passingthrough the quarter wave plate is identical to passing once through ahalfwave plate, so that the light having passed again through thehalfwave plate 14 and entering again the liquid crystal film 11 iscaused to have an electric field E₃ which is rotated by -4θ_(c) from aprincipal axis n₄ of the quarter wave plate 14. Finally, the lighthaving passed through the liquid crystal film 11 which has a molecularlong axis n₄ rotated by 6θ_(c) (=4θ_(c) +2θ_(c)) from E₃, is caused tohave an electric field E out (=E₄) which is rotated by 6θ_(c) from theaxis n₁ and is thus rotated by 8θ_(c) (=2θ_(c) +6θ_(c)) from theelectric field Ein of the incident light.

By using a reflection type arrangement as described above, a singleoptical modulation device can provide a polarization rotation anglewhich is twice that attained by using the optical modulation device inan ordinary way.

FIG. 3 is a partial sectional view of a specific embodiment of theoptical modulation illustrated by FIGS. 2A and 2B. Referring to FIG. 3,the optical modulation device has a laminate structure including, fromthe incident light side, a transparent glass substrate 301 (thickness:about 1 mm), a transparent ITO film 302 (thickness: about 1500 Å),functioning as an electrode, an insulating film 303 (thickness: about1200 Å) for preventing short circuit with a counter electrode, a rubbedpolyimide film 304 (thickness: about 200 Å) for aligning liquid crystal,a liquid crystal film 305 formed by injection into a spacing held byspacer beads having a diameter of 1-2 microns (not shown), a polyimidefilm 306 (thickness: 200 Å) similar to the polyimide film 304, a thintransparent layer 307 (i.e., of a glass plate) functioning as asubstrate for the polyimide film 306, a polymer liquid crystal film 308(thickness ≦1 micron) having a refractive index anisotropy and afunction corresponding to a quarter wave plate, a rubbed polyimide film309 (thickness: about 200 Å) for aligning the polymer liquid crystal, avapor-deposited aluminum film 310 (thickness: several microns), and aglass substrate 311 (thickness: about 1 mm).

The above-mentioned optical modulation device may be obtained by formingthe required layers respectively on the glass substrates 301 and 311 andinjecting a liquid crystal material into a space 305 formed between thesubstrates 301 and 311, followed by heat-treatment, etc., to provide abistable ferroelectric liquid crystal state. The optical modulation ofthe outgoing light may be effected by voltage application to theelectrodes through a drive circuit 312.

The optical modulation device may be provided with a multiplicity ofpixels, which can be independently controlled and easily applied toimage display. For example, each of the ITO electrode 302 and thealuminum electrode 310 may be divided into independent electrodes in theform of stripes, which are disposed to intersect each other so as toform a matrix electrode structure (such as a so-called "simple matrixdrive"). In image display using such a matrix electrode structure, it isnecessary to use a smaller pixel size in order to provide a picture ofcertain size with a higher resolution and, for example, a small liquidcrystal display device with a small diagonal size of 3 inches as usedfor a projection-type display apparatus may be formed to have a pixelsize of 60×60 microns for, e.g., EDTV.

In the arrangement of the present invention, the thickness of thequarter wave plate can be reduced to 1 micron or less by using a polymerliquid crystal having a refractive index anisotropy (Δn of about 0.2)which is remarkably larger (by one or two digits) than those of quartz,mica, stretched film, etc.

Further, the glass plate 307 disposed as a substrate for the polyimidefilm 306 may be composed of a very thin glass plate having a thicknessof, e.g., 10-100 microns, preferably 10-50 microns, since it is formedon the glass substrate 111 which has a sufficient thickness for ensuringa required device strength. The other layers between the pixelelectrodes 302 and 310 are sufficiently thinner than one pixel size, sothat the use of a thin quarter wave plate 308 of a polymer liquidcrystal and a very thin glass plate 307 is very effective for providinga high pixel area ratio (effective aperture ratio) and preventingcrosstalk between pixels.

In the operation of the above-mentioned device of the present invention,a positive-polarity pulse or a negative-polarity pulse is selectivelyapplied through the drive circuit between a pair of electrodes formed bythe ITO film 302 and the vapor-deposited aluminum film 310 so as toorient the ferroelectric liquid crystal in the liquid crystal film to afirst stable state or a second stable state corresponding to thepositive-polarity or negative-polarity pulse.

In a specific embodiment, the polymer liquid crystal film 308 was formedby using a nematic polymer liquid crystal of the following structure(PAfB): ##STR1## having a number-average molecular weight of 12520 and aweight-average molecular weight of 20744 (calculated corresponding tostandard polystyrene based on gel permeation chromatography using THFsolvent and showing a phase transition series as follows: ##STR2##

A cyclohexanone solution (10 wt. %) of the above polymer liquid crystalwas applied onto a 500 Å-thick polyimide film 309 (trade name: "SE-100",mfd by Nissan Kagaku Kogyo K.K.) by spin coating, followed by 2-3 hoursof heat treatment-at 100° C. to form a uniformly aligned polymer liquidcrystal layer in a thickness of 5500 angstroms.

In the above embodiment, the polarization axis Ein of the incidentlight, the molecular long axis n₁ in one stable state of the liquidcrystal and a principal axis n₄ of the quarter wave plate are alignedwith each other, whereby at least one state free from deviation in phasedifference even for a wavelength other than a dominant wavelength isrealized in one stable state. Accordingly, when cross nicol polarizersare used, a high contrast can be realized by suppressing thetransmittance in a black state, and when parallel nicol polarizers areused, a hue change in a white state can be suppressed.

Essentially, however, the combination of axial directions of the threefilms is not restricted and 0% or 100% modulation is possible for anycombination at least with respect to a predominant wavelength. In thisinstance, the polymer liquid crystal film 308 may preferably have athickness d in the range of 0.6-0.8 micron so as to provide an opticalpath difference Δnd of approximately λ/4.

FIG. 5 is a partial sectional view of another embodiment of the opticalmodulation device of the invention, wherein identical members aredenoted by the same reference numerals as in FIG. 3. In this embodiment,a rubbed polyimide film 306, an insulating film 512 of an insulatingmaterial, such as SiO, SiO₂ or TiO₂, and an ITO film 513 acting as anelectrode are disposed between a polymer liquid crystal film 308 and aliquid crystal film 305. A positive polarity pulse and a negativepolarity pulse are applied between the ITO films 302 and 513 from thedrive circuit 312.

According to this embodiment, the applied voltage for optical modulationcan be decreased because of a decreased spacing between the electrodes.The vapor-deposited aluminum film 310 in the figure acts as a reflectionfilm.

FIG. 6 is a partial sectional view of still another embodiment of theoptical modulation device of the invention, wherein identical membersare denoted by the same reference numerals as in FIGS. 3 and 5. In thisembodiment, an active matrix drive system using thin film transistors(TFTs) in a cell is adopted. More specifically, the optical modulationdevice according to this embodiment includes a transparent ITO electrode614, an insulating film 615 (of, e.g., SiO₂), a rubbed polyimide film616, a TFT including a gate 617 coated with an insulating film 618 ofsilicon nitride, a source 619 and a drain 620.

Hereinabove, the present invention has been explained based on severalpreferred embodiments but the present invention is also applicable inthe following manner.

(1) Ferroelectric liquid crystal devices are used in the aboveembodiments, but the present invention is also advantageously applicablegenerally to a type of devices wherein the birefringence characteristicis controlled by an electric field.

(2) A projection-type display apparatus has been explained but otherapplication are suitable, for example the invention is alsoadvantageously used for a direct watching-type display apparatus.

(3) Only two degrees of modulation, i.e., 0% and 100%, have beenexplained, but the present invention is advantageously applicable tovarious type of gradation-control systems, including a density gradationsystem using an intermediate polarization rotation angle, an arealgradation system controlling the areal ratio between 0% and 100%modulation, and a mixed system of these.

(4) An aligned polymer liquid crystal film is used as a quarterwaveplate in the above-embodiments, but quartz, mica stretched film,etc., may also be used while retaining at least the advantage of using asingle liquid crystal modulation device.

As described hereinabove, according to the present invention, thefollowing advantageous effects are specifically attained.

(i) It is possible to effect a modulation providing a large contrast byusing even a material showing a small tilt angle.

(ii) A simple device structure can be realized by adopting areflection-type arrangement, so that the production process can besimplified.

(iii) By using a polymer liquid crystal layer as a quarter wave plate orhalfwave plate, an increased aperture ratio and a minimization ofcrosstalk between pixels (leakage of light into another pixel) can berealized. Particularly, (a) a polymer liquid crystal has a largerrefractive index anisotropy than an ordinary birefringence material(such as quartz, mica, and stretched film), so that the quarter waveplate or halfwave plate can be made thinner to alleviate a decrease inaperture rate and crosstalk between pixels; and (b) the polymer liquidcrystal can be aligned at a high temperature providing a large fluidityand used in a temperature region providing a stable alignment, so thateasier handling is possible than a low-molecular weight liquid crystalsimilarly having a large refractive index anisotropy.

What is claimed is:
 1. An optical modulation device, comprising:a polarizer for polarizing input light; a first and a third film having a birefringent property and forming a first state and a second state depending on an electric field applied thereto, said first state rotating a polarization plane of said polarized light and said second state not rotating said polarization plane, said first and said third film each having a thickness which functions as a halfwave plate in said first state; and an analyzer, said optical modulation device further comprising: a second film interposed between said first film and said third film and having a birefringent property of a single state which does not rotate the polarization plane of polarized light that passed through said second state of said first film but which rotates the polarization plane of polarized light that passed through said first state of said first film, said second film having a thickness which functions as a halfwave plate when said first film is set in said first state, wherein polarized light that passed through said second film without having its polarization plane rotated passes through said second state of said third film, and polarized light having a polarization plane that rotates while passing through said second film passes through said first state of said third film before passing through said analyzer.
 2. A device according to claim 1, wherein said first film comprises a liquid crystal.
 3. A device according to claim 2, wherein said liquid crystal is a chiral smectic liquid crystal having bistability.
 4. A device according to claim 1, wherein said second film comprises a polymer liquid crystal.
 5. A device according to claim 1, which further comprises voltage application means including a pair of electrodes disposed to sandwich the first film.
 6. A device according to claim 1, which further comprises voltage application means including a pair of electrodes disposed to sandwich the first film, and means for applying a pulse of one polarity and a pulse of the other polarity selectively between the pair of electrodes.
 7. A device according to claim 1, wherein said first film has a thickness of 1.2-1.6 microns. 